Piping the Mains - Residential

Perhaps a silly question but, why not use copper ($$$) or high-temp PVC (cheap) for piping the mains? These are very low pressure systems (got mine heating the house great never exceeding 12 oz. of steam pressure!) so why not use a lighter material than iron pipe. Both copper, although expensive, and PVC would heat-up much much faster than iron - reducing initial load and loss. Seems well insulated copper or PVC would be a superior material choice for the mains? Any feedback would be greatly appreciated.